1. Field of the Invention
The present invention relates to suspensions for tandem axles and more particularly to high articulation suspensions for driven tandem axles.
2. Description of the Prior Art
A tandem axle is commonly employed to a support a heavy load such as the rear sections of heavy duty vehicles. Among suspensions commonly used with tandem axles are systems which provide a leaf or air spring for each end of the leading and trailing axles of the tandem pair and suspensions based on walking or equalizing beams. Walking beam suspensions for a tandem axle typically include two beams, each mounted perpendicularly to the axles and riding on the axles toward the outside ends of the axles. A single spring, which may be a rubber spring or steel spring, is mounted above the walking beam and below the vehicle frame and is displaced only by the average of movement of the two axles. Walking beam suspensions provide for greater axle articulation and improved traction over rough surfaces than common suspension systems and are favored for driven tandem axles used with vehicles intended for off road use. However, walking beam suspensions are inferior at maintaining a good ride for loaded and unloaded vehicles than are more commonly used systems, particularly air spring suspensions.
Good roll stability is an important factor in any vehicle suspension system. Roll stability is a particularly important factor in the design of suspensions for off road applications. The primary way to reduce roll and thereby improve roll stability is to increase the suspension's spring rate. However, simply increasing the suspension spring rate produces an increasingly harsh and uncomfortable ride. In order to keep the vertical spring rate low enough for reasonable levels of comfort it has become common practice to add auxiliary stabilizing elements to suspensions. This approach works well until the wheels on one side of a vehicle need to move independently of the roll coupled member. When this occurs the result is an increase in the effective vertical spring rate and a loss of suspension articulation. The loss of articulation in turn results in a loss of traction in rough terrain. High axle articulation and anti-roll stability have been partially incompatible objectives.
Air springs have given suspension designers some advantages in handling the problem of providing a good ride with roll stability in a single suspension design. Air can be added or released from an air spring on demand to adjust the spring's deflection as desired and keep the vehicle from listing due to uneven loading. Air springs alone, however, provide no inherent mechanism for stabilizing axle orientation and have tended to require extensive auxiliary stabilizing elements for the axles.